1. Field of the Invention
The invention relates to a sensor and method for detecting natural gas in the atmosphere.
2. Background of the Invention
Gas correlation is known to be one of the more sensitive optical measurement techniques which uses infrared emission from or absorption of a gas of interest to estimate the concentration of that gas in the sensing environment. The principle of measurement is based on a spectral correlation between a target gas of interest and a correlation gas normally the same as the target gas and contained in a cell inside the sensor. In a typical application, the correlation cell is filled with the target gas of interest to a specific pressure (i.e., concentration) to provide an optical depth at the center of the absorption lines of specific region of the molecular spectral band. The gas cell design (i.e., gas pressure and path length) is designed to obtain maximum sensitivity for the sensor near the target gas burden (i.e., the concentration times the pass length).
In general, the optimum gas cell provides a sufficiently large optical depth at the line centers of the absorption band of the gas of interest but not so large that the individual lines are well separated from each other after line broadening. When the spectral band includes a large number of absorption lines of varying strength, optimization becomes quite involved requiring numerical simulation or extensive experiments. To complicate the problem further, optimization typically involves the design of a filter bandwidth for the spectrum of interest. In general, a wide bandwidth is desired to increase the signal throughput and thus the sensitivity. While a conventional laser system may have enough brightness to be used, if it can be tuned to the desired infrared range, a conventional laser system is not suitable due to the narrow spectral line features of the laser radiation.
Passive gas correlation techniques as described by Lee and Zwick, 1985 and Ward and Zwick, 1975 (referenced below) use natural infrared radiation sources from either the ground or the sky radiance depending on the environment. The performance of passive gas correlation techniques depend on the temperature difference between the atmospheric gases and the Earth's surface. Also changes in the surface reflectivity and inhomogeneity in the background can give rise to serious errors with the passive technique and make it difficult to quantify the gas concentration in real time.
Active gas correlation techniques have been implemented in the laboratory (i.e., a stationary setup) whereby a high temperature source can be directly located in the sensor field of view. A high temperature blackbody source can be used as an active illumination source, but utilization of a blackbody source is not practical in many applications. Conventional high temperature blackbody sources have a limited emitting surface area and are substantially smaller than the field of view (FOV) or foot print of the sensor. Even though the temperature of the active source can be a magnitude or more higher than the room temperature source, the effective radiance of such a source at the infrared region does not increase significantly due to the specific nature of blackbody radiation. Thus, normally a large area low temperature blackbody source is used for active gas correlation measurement in the laboratory setting.
Active gas correlation techniques also differs from differential absorption lidar (DIAL) commonly used for remote gas measurement (Warren, 1996, Quagliano, et al, 1997, Prasad and Geiger, 1996). DIAL measurement uses two different wavelengths, one on the absorption line and one off the absorption line, provided by either two lasers or one laser whose wavelength can be switched (Lee et al, 2004, Prasad et al, 1998). Active gas correlation techniques utilize a single output pulse centered on the absorption line of the target gas. However, the spectral bandwidth of the laser should be broad enough to at least partially overlap the chosen absorption line (or lines) of the target gas, unlike the DIAL technique wherein the laser spectral bandwidth is commonly chosen to be much narrower than the width of the absorption line. One advantage of an active gas correlation technique over DIAL is that the active gas correlation sensor makes two measurements simultaneously on an identical column-content concentration of target gas, while the DIAL system's sequential measurement intrinsically probes different columns differing by the pulse sequence, different albedos for the two pulses and/or wavelengths, and speed of the platform movement. The following articles describing the development of gas correlation techniques are incorporated herein by reference in their entirety:    1. Lee, H. S. and H. H. Zwick, “Gas Filter Correlation Instrument for the Remote Sensing of Gas Leaks”, Review of Scientific Instruments, vol 56, no 9, pp 1812-1819, September 1985;    2. Ward, T. V. and H. H. Zwick, Applied Optics, vol 14, pp 2896-1536, 1975.    3. Ben-David, A., “Optimal bandwidth for topographical differential absorption lidar detection”, Applied Optics, vol 35, no 9, pp 1531-1536, 1996;    4. Warren, R. E., “Optimum detection of multiple vapor materials with frequency-agile lidar”, Applied Optics, vol 35, no 21, pp 4180-4193, 1996;    5. Prasad, N. S. and A. R. Geiger, “Remote sensing of propane and methane by means of a differential absorption lidar by topographic reflection”, Optical Engineering, vol 35, pp 1105-11, 1996;    6. Prasad, C. R., P. Kabro and S. L. Mathur, “Tunable IR differential absorption lidar for remote sensing of chemicals”, Application of Lidar to Current Atmospheric Topics III, SPIE, vol 3757, 87-95, 1998;    7. Quagliano, J. R., P. O. Stoutland, R. R. Petrin, R. K. Sander, R. J. Romero, M. C. Whitehead, C. R. Quick, J. J. Tiee and L. J. Jolin, “Quantitative chemical identification of four gasses in remote infrared (9-11 ?m) differential absorption lidar experiments”, Applied Optics, vol 36, no 9, pp 1915-1927, 1997;    8. Lee, H. S., C. R. Prasad and J. Zhang “Tunable IR laser for MALDI”, U.S. Pat. No. 6,683,984, (2004); and    9. Sandsten, J., H. Edner and S. Svanberg, “Gas imaging by infrared gas-correlation spectrometry”, Optics Letters, 21, 23, 1945-1947, 1996.